The present invention concerns an arrangement in connection with equipment for cooling billets, preferably of aluminium, comprising a housing with openings for axial passage of the billet through the housing as well as an internal cooling ring with supply lines for a cooling medium.
The maximum extrusion speed depends, among other things, on the temperature of the billet before the start of the extrusion process, as well as the, alloy and the prior temperature history of the billet. The prior temperature history for AlMgSi alloys is significant because it affects the content of MgSi phases in the billet. It is generally known that large quantities of MgSi phases present in the billet before the start of the extrusion operation will result in a poorer quality of extrudate and a lower maximum extrusion speed.
In the applicant""s own European Patent No. 0302623, a method is described for the production of an aluminium alloy for extrusion purposes in which the alloy undergoes a certain heat treatment before cooling, immediately before extrusions of the alloy, to avoid the MgSi phases.
The cooling immediately before extrusion is performed using cooling equipment arranged in connection with the extrusion equipment.
U.S. Pat. No. 5,027,634 describes cooling equipment in which the aluminium billet is designed to be passed through a cooling ring with two annular nozzles for the supply of the cooling liquid along the full circumference of the billet. This solution has proved to produce uneven cooling along the circumference and thus a temperature gradient over the cross-section of the billet. In turn, this has the result that, in extrusion equipment in which several extrudates are extruded through multi-aperture extrusion tools, the extrudates are pressed out at different speeds with different qualities.
It is otherwise common to produce a temperature difference or temperature gradient in the longitudinal direction of a billet before extrusion in order to achieve consistent quality over the full length of the extrudate. The temperature gradient is created to compensate for the heat which is generated during the extrusion process. More precisely, the billet is cooled so that the temperature of the end which is closest to the extrusion tool is highest while the other end, which is furthest away from the tool, is the coolest. This cooling can be adapted so that, depending on the extrusion speed, etc., the temperature in the extrudate at the outlet of the extrusion nozzle is always the same.
For example, U.S. Pat. No. 2,639,810 describes a solution in which the billet, before extrusion in a press, is cooled so that a temperature gradient is formed between the ends of the billet. The temperature gradient can, in accordance with the patent specification, be achieved by spraying the billet or by dipping one end of the billet in water.
However, the latter prior art cooling equipment solution also entails a disadvantage for the billet, namely that the cooling along the circumference, and thus over the cross-section of the billet, is uneven and uncontrolled.
This has also been confirmed in tests in which measurements were taken at four points along the periphery of a billet immediately after cooling of the billet through a cooling ring in which cooling water was supplied evenly along the periphery through a gap. The tests showed that the temperature difference between the top and bottom of the billet could be as much as 40-50xc2x0 C. and that the top was coldest and the bottom hottest.
At first sight, it seems somewhat surprising that the top is coldest as one would have expected gravitational force to have resulted in a greater collection of water against the bottom of the billet and thus increased cooling of the bottom. However, on closer observation, the effect seems to be caused by a combination of greater spread and longer cooling exposure time for the water on the top of the billet as well as boiling and thus partial formation of a vapor barrier layer against the bottom of the billet.
The present invention is directed to an arrangement in connection with the cooling of billets in which the above problems are greatly reduced or completely eliminated.
The present invention is characterized in that the billet is designed to be supplied with a cooling medium so that even cooling is achieved, and temperature gradient around the circumference of the billet is avoided.